Quarter wave retardation films (QWFs, ¼ waveplates or λ/4 films) are known in prior art. A QWF creates a phase difference of π/2 in two linearly polarized light rays having vibration planes perpendicular to each other, and thus convert for example linearly polarized light into circular polarized light or vice versa. It typically consists of a polymer film that is stretched uniaxially to give anisotropic optical properties, and can be used as optical phase retarders for example in liquid crystal displays (LCD) or other optical systems. However, as the retardation of a conventional QWF, for example made of stretched polycarbonate (PC), varies with the wavelength of light, it can be used only in a narrow wavelength range
Achromatic QWFs (AQWFs) are also known in prior art. An AQWFs (or broad band QWF) converts linearly polarized light into circular polarized light at all visible wavelengths, which is a considerable advantage for many applications. For example, if an AQWF is used instead of conventional QWF in a reflective or transflective LCD, the contrast ratio of the LCD increases sharply.
In prior art several methods are known how to create an AQWF. For example, stretched ½ and ¼ waveplates of materials with different birefringence dispersion were laminated with their optical axis orthogonal to each other to form an AQWF (for example in S. Pancharatnama, The proceedings of the Indian Academy of Science, Vol. XLI, No. 4, Sec. A, 1955). Furthermore, an AQWF comprising stretched ¼ waveplate films made of a mixture of positive and negative photoelastic polymers has been described (A. Uchiyama, T. Yatabe; SID 01 Digest, 566, 2001). Also, stretched ½ and ¼ waveplate films made of materials with the same birefringence dispersion were arranged with their optical axis at an angle of 60° ti give an AQWF (M. Okamoto, K. Minoura, S. Mitsui, IDW99, 49, 1999). Furthermore, AQWFs comprising laminated waveplates comprising one or more stretched ½ waveplates and one or more stretched ¼ waveplates arranged at various angles have been disclosed for example in JP-A-05-100114, U.S. Pat. No. 6,025,958 and EP-A-0 989 441.
However, the performance of the AQWFs of prior art is still not satisfying for specific applications. Also, QWFs of stretched polymer films often have only moderate birefringece and therefore require high thickness to give the necessary optical retardation. In particular in case of laminated waveplates the use of more than two films is expensive, raises manufacturing problems and leads to undesirable high thickness of the AQWF, which is a disadvantage especially for use in flat panel LCDs.